High-shrinkage acrylic fiber, spun yarn containing the same, and step pile fabric using the spun yarn

ABSTRACT

Provided are an acrylic fiber suitable for a step pile fabric such as a high-pile textile, a spun yarn containing said fiber, and a pile fabric in which said spun yarn is used. In the present invention, the acrylic fiber is a high-shrinkage acrylic fiber having a single-filament fineness of 1-7 dtex, a shrinkage of 20-40%, and post-shrinking bulkiness of 0.19×10 1 -0.30×10 1  mm 3 /g; or an acrylic fiber having a single-filament fineness of 2-7 dtex, elongation of 50-70%, and bulkiness of 0.19×10 1 -0.30×10 1  mm 3 /g.

TECHNICAL FIELD

The present invention relates to acrylic fibers suitable for producing step pile fabrics, spun yarns containing the acrylic fibers, and step pile fabrics produced using the spun yarns.

BACKGROUND ART

Since natural furs are generally made of napped fibers with the tip narrower than the base, they have a unique soft texture despite their resilience. In recent years, natural furs are less likely to be used from the standpoint of environmental protection. Thus, development of napped products using synthetic fibers to obtain a texture similar to that of natural furs has been in great demand.

Among synthetic fibers, especially acrylic fibers exhibit animal-fur like texture and luster. Thus, they are widely used as pile material for producing animal fur-like napped products.

Various types of napped products formed with acrylic fibers to imitate natural furs have been commercially available. For those faux products, there is no choice but to use fibers with a uniform thickness. Thus, if the fiber is made to have the same thickness as that of the base, the product is resilient but has a rough texture, and a thickness that is the same from the base to the tip causes the texture to lack resilience.

As for the technique for producing napped products using synthetic fiber to exhibit a texture similar to that of natural furs, one example is a step pile fabric consisting of long-pile and short-pile portions. The long-pile portion is formed to imitate a structure of animal guard hairs and the short-pile portion to imitate a structure of down hairs. Thus, step pile fabrics are suitable for producing fabrics with animal fur-like textures.

It is necessary that the long-pile portion seldom becomes entangled and provides a soft touch, and that the short-pile portion exhibits nap, volume and bulkiness. Especially in demand is development of acrylic fibers capable of securing excellent nap and bulkiness in the short-pile portion while maintaining a soft texture in the entire pile fabric.

As an example of techniques to produce napped products using synthetic fibers so as to achieve a similar texture to that of natural furs, super flat acrylic fibers are proposed in JPH8-260234A (Patent Literature 1), for example; the acrylic fibers have protrusions formed continuously on the long side of a fiber in the fiber axial direction, and are used as the guard hair component.

Also, JPH9-78375A (Patent Literature 2), for example, describes fibers for pile material and the like, which are formed with super flat acrylic fiber with a flatness degree of 15˜30 and a single-filament fineness of 2˜3 denier (2.2˜3.3 dtex), shrinkable acrylic fiber with a single-filament fineness of 1˜5 denier (1.1˜5.6 dtex), and other acrylic fibers.

It is important that napped products have excellent nap that is less likely to flatten and that the napped portions seldom become entangled. So far, the touch of napped products has been improved by forming the guard-hair component with fibers having a flat cross section, dog-bone cross section, broad-bean cross section, circular cross section and the like. However, none is effective enough to enhance the nap.

Moreover, in another example described in JPH11-350298A (Patent Literature 3), shrinkable fibers having a round cross section are mainly used as the down hair component, and flat fibers are blended in the product.

However, there is no conventional art that describes shrinkable fibers having other cross-sectional shapes.

PRIOR ART LITERATURE Patent Literature

Patent Literature 1: JPH8-260234A

Patent Literature 2: JPH9-78375A

Patent Literature 3: JPH11-350298A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The inventors of the present invention have conducted extensive studies in consideration of the above-mentioned problems, and have found that nap properties are improved when high-shrinkage fibers with modified cross-sectional shapes are used for the down hair component.

Solutions to the Problems

An acrylic fiber related to the present invention is a high-shrinkage acrylic fiber that has a single-filament fineness of 1˜7 dtex, a shrinkage rate of 20˜40%, and a post-shrink bulkiness of 0.19×10¹˜0.30×10¹ mm³/g.

The acrylic fiber related to the present invention has a single-filament fineness of 2˜7 dtex, an elongation rate of 50˜70%, and a bulkiness of 0.19×10¹˜0.30×10¹ mm³/g.

In the acrylic fiber related to the present invention with such an elongation rate and bulkiness specified as above, a minimum value of the second moment of area per unit fineness is preferred to be at least 600 μm⁴ in a direction where the second moment of area is smallest, and a maximum value of the second moment of area per unit fineness is at least 2200 μm⁴ in a direction where the second moment of area is greatest.

The minimum value of the second moment of area is more preferred to be at least 4000 μm⁴ in a direction where the second moment of area is smallest.

In addition, the acrylic fiber related to the present invention is preferred to satisfy any of requirements (I), (II) and (III) described below:

(I) The cross section of a fiber is formed with a circle and rectangle, where the rectangle having a short side shorter than the diameter of the circle penetrates through the circle in such a way that both ends of the rectangle are positioned outside the circle: when the length of the long side of the rectangle is set as “a” and the length of the short side as “b,” “a/b” is 3˜25; and when the diameter of the circle is set as “W” and the shorter height of the maximum heights respectively measured from each of both long sides of the rectangle inside the circle is set as “H,” “W” is a/10˜4a/5, and “H” is b/2˜2b.

In the following, the shape of such a fiber cross section may also be referred to in the present application as a “UFO cross section.”

(II) The cross section of a fiber has a triangular shape: when the length of the longest side of the triangle is set as “C” and the height from the longest side positioned at the base of the triangle is set as “B,” B/C is 0.5 or greater.

(III) The cross section of a fiber has a dumbbell shape and the degree of narrowing (X/Y) is 1.1˜2.5.

“X” is the maximum length of major diameter between narrowed ends. “Y” is the length at the narrowest portion.

An acrylic fiber related to the present invention is preferred to be made of an acrylonitrile-based polymer containing an acrylonitrile unit at 50 mass % or greater.

The spun yarn related to the present invention contains the aforementioned acrylic fiber of the present invention at 20˜50 mass %.

A step pile fabric related to the present invention consists of a long-pile portion and a short-pile portion. The short-pile portion contains the aforementioned acrylic fiber related to the present invention, and its rate of recovery from compression is 35˜90.

In a step pile fabric related to the present invention, the content of the aforementioned acrylic fiber contained in the short-pile portion is preferred to be 20˜50 mass % of the entire pile portion.

In a step pile fabric related to the present invention, the base of pile is preferred to be in the initial state of spun yarn.

The short-pile portion of a step pile fabric related to the present invention is preferred to have a pile length of 5˜20 mm.

The long-pile portion of a step pile fabric related to the present invention is preferred to have a pile length of 6˜40 mm.

The material for the long-pile portion of a step pile fabric related to the present invention is not limited specifically, and may be made of synthetic fibers or natural fibers selected according to the desired texture to be obtained.

Among those fibers, it is preferred to contain any one or more fibers selected from among acrylic fibers, polyester fibers and animal fibers. Acrylic fibers are preferred, since crimp is easier to stretch when heat is applied during the finishing process of pile, and a soft texture and an excellent lustrous appearance are obtained.

Since polyester fibers are resilient, they are preferred when a fabric with a rougher texture is desired.

It is preferred to use animal fibers in long piles, since the texture and appearance are made similar to those of natural material.

In a step pile fabric related to the present invention, the content of one or more fibers selected from among acrylic fibers, polyester fibers and animal fibers is preferred to be 50˜100 mass % of the entire long-pile portion.

The content is preferred to be 50 mass % or greater, more preferably 80 mass % or greater, since it is easier to achieve the characteristics specific to that material.

The long-pile portion of a step pile fabric related to the present invention is preferred to have a single-filament fineness of 1˜50 dtex.

A single-filament fineness of 1 dtex or greater makes it easier to obtain a soft texture, and a single-filament fineness of 50 dtex or less makes it easier to obtain excellent nap properties in the pile fabric.

A single-filament fineness of 2˜25 dtex is more preferred, even more preferably 3˜10 dtex, since spinning stability is excellent and it is easier to stretch a crimp by applying heat during a polishing process.

In a step pile fabric related to the present invention, the cross-sectional shape in a fiber axial direction of acrylic fiber or polyester fiber used for the long-pile portion may be selected appropriately from a flat shape, Y shape, UFO shape, dumbbell shape, round shape or the like according to the desired texture.

Among those shapes, a flat shape, Y shape, UFO shape or dumbbell shape is preferred since such a shape makes it easier to obtain a texture similar to that of animal furs.

In a step pile fabric related to the present invention, the difference in length between long-pile and short-pile portions is preferred to be 1˜20 mm. If a difference is 1 mm or greater, characteristics specific to the long-pile portion are more likely to be obtained, whereas a difference of 20 mm or less makes it easier to obtain a step pile fabric with excellent nap that recovers well when compressed.

From the aforementioned viewpoints, the difference in length between long-pile and short-pile portions is more preferred to be 3˜15 mm, even more preferably 5˜10 mm.

Effects of the Invention

According to the present invention, step pile fabrics with excellent nap that recovers well when compressed are obtained, and acrylic fibers suitable for producing such step pile fabrics are also obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view perpendicular to the fiber axis of high-shrinkage acrylic fiber (I) related to the present invention;

FIG. 2 is a cross-sectional view perpendicular to the fiber axis of high-shrinkage acrylic fiber (II) related to the present invention;

FIG. 3 is a cross-sectional view perpendicular to the fiber axis of high-shrinkage acrylic fiber (III) related to the present invention; and

FIG. 4 is a perspective view seen from diagonally above when an acrylic fiber bundle related to the present invention is measured by using a bulkiness measuring instrument.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, the present invention is described in further detail.

<Acrylonitrile-Based Polymer>

An acrylonitrile-based polymer used in the embodiments of the present invention is made of acrylonitrile and an unsaturated monomer polymerizable with the acrylonitrile. Examples of such unsaturated monomers are acrylic acid, methacrylic acid and their alkyl esters, vinyl acetate, acrylamide, vinyl chloride and vinylidene chloride. In addition, depending on usage purposes, ionic unsaturated monomers may also be used, for example, sodium vinylbenzensulfonate, sodium methallyl sulfonate, sodium allyl sulfonate, sodium acrylamide methylpropane sulfonate, sodium para-sulfophenyl methallyl ether and the like.

The amount of an acrylonitrile unit contained in a polymer is preferred to be 50 mass % or greater, more preferably 80 mass % or greater, even more preferably 90 mass % or greater, while its upper limit is preferred to be 99 mass % or less.

When a polymer contains an acrylonitrile unit at 80 mass % or greater, the resultant fiber is strong enough to be used. When the content of the acrylonitrile unit in the polymer is 99 mass % or less in the polymer, excellent results are achieved when the fiber is dyed.

The acrylonitrile-based polymer of an acrylic fiber related to the present invention may be made of one type of a polymer, or may be mixed with two or more polymers containing different amounts of acrylonitrile.

Suspension polymerization, solution polymerization or the like may be selected as a method for polymerizing the acrylic polymer, but any other method may also be employed. The molecular weight of the acrylic polymer is not limited specifically as long as it is within a range generally used for producing acrylic fibers. However, when the acrylic polymer is made into a 0.5 wt % dimethylformamide solution, the reduced viscosity at 25° C. is preferred to be in a range of 1.5˜3.0 in view of spinning stability and fiber strength.

<Spinning Dope>

A spinning dope is prepared by dissolving an acrylic polymer in a solvent to have a concentration of 15˜28 mass %. When the concentration is at least 15 mass %, the cross-sectional shape of the fiber is not so different from the shape of the discharge port of a spinning nozzle when it is coagulated, and a desired cross-sectional shape is easier to obtain. On the other hand, a concentration of no greater than 28 mass % is preferred, since the chronological stability of the spinning dope is excellent, and spun results are most likely to be stable.

As for the solvent, in addition to organic solvents such as dimethylformamide, dimethylacetamide, and dimethylsulfoxide, nitric acid, a Rhodanate solution, zinc chloride solution or the like may also be used. However, to control the cross-sectional shape of a fiber by the shape of the discharge port of a spinning nozzle, it is more effective to use an organic solvent. Considering the speed of coagulation and ease of handling, dimethylacetamide is more preferred.

<Spinning>

When a coagulated fiber is drawn out of a nozzle, if drafting, which is defined by the ratio of the take-off speed of the coagulated fiber relative to the discharge linear velocity of the spinning solution, is set to be 0.7˜2.0, it is easier to obtain an acrylic fiber with a modified cross-sectional shape that is substantially similar to the shape of a discharge port of a spinning nozzle. A drafting of 0.7 or greater is preferred, since the cross-sectional shape of the fiber is not so different from the shape of the discharge port of a spinning nozzle during coagulation, and a desired cross-sectional shape is more likely to be obtained. On the other hand, it is preferred to be 2.0 or less, since fiber breakage in a coagulation bath is less likely to occur.

The obtained coagulated fiber is stretched, washed and dried by conventional methods and conditions. Then, after a relaxation process, the fiber is stretched in a steam-stretching process to be 1.10˜2.00 times longer so as to obtain a thermally shrinkable acrylic fiber with a shrinkage rate of 20˜40% according to the present invention.

To form raw stock, the fiber is cut into a predetermined length according to usage purposes. The raw stock is processed into a pile fabric through a conventionally known pile processing method.

The acrylic fiber of the present invention is shrinkable when wet heat or dry heat is applied thereon.

First, pre-shrink acrylic fibers are described.

<Single-Filament Fineness>

One of the main purposes of an acrylic fiber related to the present invention is to be contained in the short-pile portion of a step-pile fabric. To achieve a soft texture in a step pile fabric, the single-filament fineness of the acrylic fiber before it shrinks is preferred to be 1˜7 dtex.

A single-filament fineness of at least 1 dtex makes it easier to obtain excellent nap properties when formed into a step pile fabric, while a single-filament fineness of no greater than 7 dtex makes it easier to obtain a soft texture when formed into a step pile fabric. Considering those viewpoints, the single-filament fineness is more preferred to be 1˜5 dtex and even more preferably 2˜4 dtex.

<Shrinkage Rate of Acrylic Fiber>

As for the shrinkage rate of an acrylic fiber related to the present invention, it is sufficient if the shrinkage rate when exposed to wet heat (processed 3 minutes in boiling water) or when exposed to dry heat (processed 10 minutes at 130° C.) is 20˜40%.

A shrinkage rate of at least 20% means sufficient shrinkable properties, while bulkiness and design effects of pile portions are achieved in step pile fabrics. Also, if the shrinkage rate is no greater than 40%, the texture of a step pile fabric maintains softness, and excellent product quality is thus secured.

From those viewpoints, the shrinkage rate is more preferred to be 25˜35%, even more preferably 27˜33%.

<Post-Shrink Bulkiness>

Post-shrink bulkiness of an acrylic fiber related to the present invention is 0.19×10¹˜0.30×10¹ mm³/g.

Bulkiness of at least 0.19×10¹ mm³/g makes it easier to form a step pile fabric with excellent nap and volume. If the bulkiness is no greater than 0.30×10¹ mm³/g, the texture of a step pile fabric maintains softness, and excellent product quality is thus secured. From those viewpoints, the bulkiness is more preferred to be 0.21×10¹˜0.25×10¹ mm³/g.

The following is a description of post-shrink acrylic fibers related to the present invention.

<Single-Filament Fineness>

One of the main purposes of an acrylic fiber related to the present invention is to be contained in the short-pile portion of a step-pile fabric. To achieve a soft texture in a step pile fabric, the acrylic fiber is preferred to have a post-shrink single-filament fineness of 2˜7 dtex.

A single-filament fineness of at least 2 dtex is preferred, since the rate of recovery from compression is excellent when the fiber is used in the short-pile portion of a step pile fabric, while a single-filament fineness of no greater than 7 dtex is preferred because that makes it easier to maintain a soft texture. Considering those viewpoints, the single-filament fineness is more preferred to be 2˜5 dtex and even more preferably 2˜4 dtex.

<Elongation Rate>

The elongation rate of an acrylic fiber related to the present invention is 50˜70%. An elongation rate of at least 50% is preferred since it is easier to form a step pile fabric with excellent nap properties. On the other hand, an elongation rate of no greater than 70% is preferred, since the density of the step pile fabric will not increase too much and stiff texture is prevented.

From those viewpoints, the elongation rate is more preferred to be 52˜65%, even more preferably 56˜63%.

<Second Moment of Area>

The second moment of area means the degree of difficulty to deform an object in question relative to a bending moment. A higher value of the second moment of area means that the object is less likely to bend, while a lower value means the object is more likely to bend.

In addition, when an object has a modified cross section, the value of the second moment of area varies depending on the direction of bending.

Regarding the acrylic fiber related to the present invention, the minimum value of the second moment of area per unit fineness is preferred to be 600 μm⁴ or greater in a direction where the second moment of area is smallest; and the maximum value of the second moment of area per unit fineness is preferred to be 2200 μm⁴ or greater in a direction where the second moment of area is greatest.

If the minimum value of the second moment of area per unit fineness is at least 600 μm⁴ in a direction where the second moment of area is smallest, and when the fiber is used in the short-pile portion of a step pile fabric, the nap is less likely to be flattened under external force; even if the pile is flattened, the nap is more likely to recover from compression. From those viewpoints, the minimum value is more preferred to be 1000 μm⁴ or greater, even more preferably 4000 μm⁴ or greater.

If the maximum value of the second moment of area per unit fineness is at least 2200 μm⁴ in a direction where the second moment of area is greatest, and when the fiber is used in the short-pile portion of a step pile fabric, the nap is less likely to be flattened under external force; even if the pile is flattened, the nap is more likely to recover from compression. From those viewpoints, the maximum value is more preferred to be 6000 μm⁴ or greater, even more preferably 10000 μm⁴ or greater.

When a step pile fabric is formed, since the directions of fiber cross-sections of nap fibers vary, the higher the maximum value of the second moment of area per unit fineness is in a direction where the second moment of area is greatest, the less likely is the nap to be flattened even when external force is exerted in a certain direction.

The minimum value of the second moment of area per unit fineness in a direction where the second moment of area is smallest is preferred to be at least 4000 μm⁴, since the nap is less likely to be flattened under external force when the fiber is used in the short-pile portion of a step pile fabric.

<Cross-Sectional Shape of Acrylic Fiber>

The cross-sectional shape of an acrylic fiber related to the present invention is not limited specifically. The following are examples.

Requirement (I) is as follows.

The fiber cross section is formed with a circle and rectangle, where the rectangle having a short side shorter than the diameter of the circle penetrates through the circle in such a way that both ends of the rectangle are positioned outside the circle. When the length of the long side of the rectangle is set as “a” and the short side as “b,” a/b is 3˜25; when the diameter of the circle is set as “W” and the smaller value of the maximum heights respectively measured from each of the long sides of the rectangle positioned inside the circle is set as “H,” “W” needs to be a/10˜4a/5, and “H” needs to be b/2˜2b. In the present application, a fiber cross section indicates the cross section of a fiber perpendicular to the longitudinal direction of the fiber.

When an acrylic fiber that satisfies requirement (I) is shrunk, such a cross-sectional shape contributes to achieving excellent resilience and nap properties.

Requirement (II) is as follows.

A fiber cross section has a triangular shape; in such a triangle, when the length of the longest side among three sides of the triangle is set as “C” and the height from the longest side positioned at the base is set as “B”, then B/C needs to be 0.5 or greater. If B/C is less than 0.5, the triangle becomes a narrow flat shape, and desired effects are not achieved since the resilience properties of the fiber cross section are reduced. In addition, the maximum value of B/C is 0.87 when it is an equilateral triangle. Moreover, it is an option for the cross section to be an approximate triangle where the angles are slightly rounded.

Requirement (III) is as follows.

The single fiber cross section is shaped like a dumbbell, where the maximum length of major diameter between narrowed ends is set as “X” and the length at the narrowest portion is set as “Y,” the degree of narrowing (X/Y) needs to be 1.1˜2.5. When the degree of narrowing is at least 1.1, the fiber cross section is closer to a regular flat cross section, and desired resilience and nap properties are achieved. If the degree of narrowing is 2.5 or less, a decrease in spinning stability and a reduction in fiber strength are suppressed.

<Spun Yarn>

The acrylic fiber related to the present invention forms the short-pile portion of a step pile fabric. The acrylic fiber is blended with other fibers at a predetermined rate and is formed into a yarn by a conventional method. The rate of blending the acrylic fiber related to the present invention is preferred to be 20˜50 mass %.

When the blending rate is 20 mass % or greater, the volume of a step pile fabric is not lowered, and when the rate is 50 mass % or less, the soft texture of a step pile fabric is maintained.

When blending into a step pile fabric related to the present invention, in addition to a flat acrylic fiber, non-shrinkable acrylic fiber and shrinkable acrylic fiber, other fibers may also be used; for example, fibers of 1.5 denier or lower to increase the softness, or fibers with specific functions to provide antibacterial or antistatic properties. Fibers to be blended in are not limited to any particular type.

A step pile fabric may be produced by employing any known methods such as a method for producing a pile fabric from spun yarn and a method for producing a pile fabric by high-pile knitting.

When the high-shrinkage acrylic fiber related to the present invention is used, it is preferred to be formed into spun yarn, from which pile is produced, because such a method makes it easier to form different lengths in the short-pile and long-pile portions respectively.

<Step Pile Fabric>

In a step pile fabric related to the present invention that consists of a long-pile portion and a short-pile portion, the short-pile portion contains the acrylic fiber of the present invention, and the later-described rate of recovery from compression is set to be 35˜90.

When the acrylic fiber related to the present invention is used in the short-pile portion, a step pile fabric is achieved which has excellent nap properties and rate of recovery from compression.

When a step pile fabric related to the present invention is set to have a rate of recovery from compression at 35 or greater, even if the nap of the fabric is flattened when compressed by external force, the nap is more likely to recover. The maximum value of the rate of recovery from compression is 90.

The rate of recovery from compression is more preferred to be at least 38, even more preferably at least 43.

In the short-pile portion of a step pile fabric related to the present invention, the acrylic fiber described above is preferred to be contained at 20˜50 mass % of the entire pile portion. When the content of the acrylic fiber related to the present invention is at least 20 mass % of the entire pile portion, the nap of the long-pile portion is excellent, and desired resilience and nap properties are achieved. If the content is no greater than 50 mass %, the blending ratio relative to the long-pile portion is not too low, thus excellent texture is achieved in the long-pile portion. Accordingly, a texture similar to that of natural fur is obtained.

The content is more preferred to be 30˜45 mass %. Considering the above viewpoints, 35˜40 mass % is preferred.

In a step pile fabric related to the present invention, the base of pile is preferred to maintain the initial state of spun yarn. If the base of pile is in the initial state of spun yarn, the fiber bundle is less likely to collapse at the base. Thus, a step pile fabric with excellent nap properties is formed.

The length that maintains the initial state of spun yarn is preferred to be 1˜5 mm, considering nap properties.

In a step pile fabric related to the present invention, the length of the short-pile portion is preferred to be 5˜20 mm. A short-pile portion having a length of at least 5 mm maintains excellent nap properties, and a short-pile portion having a length of no greater than 20 mm provides soft texture for the step pile fabric.

The length of a short-pile portion is preferred to be 5˜10 mm, considering the above features.

In a step pile fabric related to the present invention, the difference in lengths of the short-pile and long-pile portions is preferred to be 1˜20 mm. Such a range is preferred because the step pile fabric can achieve both the nap properties and texture similar to those of natural furs.

Moreover, to maintain its firmness while resilience and nap properties are expressed, the pile length of a long-pile portion is especially preferred to be 6˜40 mm in a step pile fabric related to the present invention. If the pile length of a long-pile portion is no greater than 40 mm, the firmness and resilience of acrylic fiber prevent fiber tips from converging.

EXAMPLES

In the following, the present invention is described in further detail by referring to the examples. Measurement of each property in the examples was conducted as follows.

<Measurement of Single-filament Fineness>

Using an autovibro-type fineness measuring instrument (Denier Computer DC-11, made by Search Control Electric K.K.), single-filament fineness is measured under conditions of temperature of 25° C. and humidity of 65%. Measurement is conducted 25 times, and the average value is employed.

<Measurement of Shrinkage Rate>

-   (1) An acrylic fiber bundle is cut into an approximate length of 2     meters, and one end of a piece is fixed from which the piece is     dangled in a vertical direction. To the other end, a load of 10     mg/dtex per unit fineness is attached. Two points of the piece are     marked to have a vertical distance of 100 cm (L1) between them. -   (2) The acrylic fiber bundle is placed in boiling water for 3     minutes under conditions of no tension. -   (3) The same load as in (1) is applied on the acrylic fiber bundle,     and the distance (L2) (cm) is measured between the two points marked     in (1). -   (4) The shrinkage rate is calculated by the following formula.

Shrinkage rate (%)={(L1-L2)/L1}×100

The measurement process above is repeated three times, and the average value is employed as the shrinkage rate.

<Measurement of Bulkiness>

-   (1) Using a measuring jig 1 shown in FIG. 4, the bulkiness of an     acrylic fiber bundle is measured as follows. As shown in FIG. 4, the     measuring jig 1 is formed with first and second board members 11, 12     vertically positioned to be parallel with a space between them, a     jig main body 10 connected by third board member 13 to the lower     edges of two board members 11, 12, and rectangle columnar weight 14     positioned to be movable in the space between first and second board     members 11, 12. According to the present embodiment, the space     between two board members 11, 12 is 10 mm, and the depth of jig main     body 10 and the length of weight 14 are each set to be 40 mm.

To measure the bulkiness of an acrylic fiber bundle, acrylic fiber bundles 15 formed of n acrylic fibers as one bundle are arranged on the upper surface of third board member 13. Here, “n” is set at 500˜800. The fiber portions protruding from jig main body 10 are cut off so as to line up edges of fiber bundles on both ends.

-   (2) On the entire upper portion of the acrylic fiber bundles, weight     14 is applied to exert 0.196 N load. After the load is kept thereon     for one minute, height “H” cm (H×10¹ mm) and mass “W” (g) of fiber     bundles 15 are measured. -   (3) Bulkiness is calculated by the following formula.

Bulkiness (mm³/g)=10×40×H/(N×W)

The measurement is conducted 10 times, and the average value is employed as the bulkiness (mm³/g).

<Minimum Value, Maximum Value of Second Moment of Area per Unit Fineness>

The second moment of area of each cross-sectional shape is measured, and directions of X- and Y-axes of each cross-sectional shape are changed appropriately to obtain the minimum and maximum values of each second moment of area.

Moreover, values obtained as minimum and maximum values of the second moment of area are divided by single-filament fineness, and the calculated values are set to be the minimum and maximum values of the second moment of area per unit fineness.

<Measurement of a, b, W, H, B, C, X, Y and Z at Fiber Cross Section>

Using a scanning electron scope (S-3500N, made by Hitachi High-Technologies Corporation), 25 fiber cross sections with gold deposited by an ion coater (type IB-3, made by Eiko Engineering Co., Ltd.) are observed at a magnification of 500 times to measure lengths of a, b, W, H, B, C, X, Y and Z respectively, and their average values are each calculated. Here, Z refers to the length between the edges of narrowed ends at the major axis.

<Rate of Recovery from Compression in Step Pile Fabric>

The rate of recovery from compression is evaluated as follows. A step pile fabric is cut into a 3 cmx3 cm piece, and the piece with a load of 141 g/cm² applied thereon is left standing for three days in a dryer with a temperature set at 35° C. After being left standing for three days, angle “A” of a nap is measured immediately after the load is removed. Furthermore, the same piece is left standing for one day in room temperature (25° C., relative humidity of 65%), and angle “B” of the same nap is measured. Angles “A” and “B” are those measured between the horizontal surface of a fabric piece and the direction of nap (acute angles). The rate of recovery from compression observed in the nap of a step pile fabric is calculated by (B-A). The rate of recovery from compression means the amount of change of nap angles observed in a day after a load is removed. The measurement is conducted 10 times, and the average value is employed as the rate at which the step pile fabric recovers from compression.

<Texture Evaluation of Step Pile Fabric>

Sensory testing is conducted by visually observing and by touching a fabric. Three expert researchers evaluated the following three categories of each fabric on a scale of 1 to 5 below.

-   Categories to be evaluated: looseness, softness, bulkiness -   Each property exhibited on the surface of a step pile fabric is     evaluated as -   5: significantly excellent, -   4: excellent, -   3: normal, -   2: poor, or -   1: significantly poor.

Example 1

A copolymer consisting of 90 mass % of acrylonitrile and 10 mass % of vinyl acetate was obtained through solution suspension polymerization. A 0.5 mass % dimethylformamide solution of the polymer had a reduced viscosity of 2.0 at 25° C. The polymer was dissolved in dimethylacetamide to prepare a spinning dope with a polymer concentration of 24 mass %. From the discharge port of a spinning nozzle with a cross-sectional shape specified in Table 1, the spinning dope was discharged into a dimethylacetamide solution with a solvent concentration of 40% to obtain coagulated fiber. The coagulated fiber was further stretched in hot water to be five times as long, was washed, and was dried with a drying roll. Then, the fiber was subjected to a heat relaxation process in a compressed steam ambient. After that, the fiber was stretched in a steam stretching process to be twice as long, and was crimped mechanically. Accordingly, an acrylic fiber was obtained. The shrinkage rate of the acrylic fiber is shown in Table 1.

Next, an acrylic fiber bundle was cut into an approximately 2 m-long sample. The sample was placed into a container, and the acrylic fiber was set to shrink for 3 minutes in the container under a flow of 100° C. steam. The shape and physical properties of post-shrink acrylic fiber are shown in Tables 1 and 2.

Tables 1 and 2 show the post-shrink shape and various physical properties of acrylic fibers except for shrinkage rates.

A pre-shrink acrylic fiber was cut into various lengths ranging from 76˜127 mm. Blended fibers were prepared by using 40 mass % of the acrylic fiber and 60 mass % of acrylic fiber with a flat fiber cross section (flatness degree of 7) (product number H155 BRE3.3 TVCL, single-filament fineness of 3.3 dtex, variable cut fiber length of 76˜127 mm, shrinkage rate of 0%, made by Mitsubishi Rayon). The blended fibers were subjected to regular worsted spinning, and a spun yarn with a yarn count of 2/28 Nm was obtained. The spun yarn was subjected to a process for creating bulkiness and to a skein dyeing. After the yarn was knitted and cut, known pile procedures such as brushing, polishing and shearing were conducted to obtain a step pile fabric. Evaluation results of the step pile fabric are shown in Table 3.

Examples 2-5, Comparative Examples 1, 2

Acrylic fibers were each prepared the same as in Example 1 except that the cross-sectional shape of the discharge port of a spinning nozzle was changed to obtain a post-shrink cross-sectional shape of acrylic fiber as shown in Table 1. Accordingly, acrylic fibers as shown in Tables 1 and 2 were obtained.

Step pile fabrics were each formed the same as in Example 1 except that fibers in the short-pile portion were respectively changed to the fibers obtained above. Evaluation results of step pile fabrics are shown in Table 3.

Comparative Example 3

Acrylic fiber was prepared the same as in Example 1 except that the cross-sectional shape of the discharge port of a spinning nozzle was changed and no steam stretching was conducted. Accordingly, acrylic fiber as shown in Tables 1 and 2 was obtained. The fiber was cut into 38 mm lengths.

A step pile fabric was obtained as follows: a sliver was formed by blending 40 mass % of the above acrylic fiber with an acrylic fiber having a flat fiber cross section (flatness degree of 7) (product number: H155 BRE3.3 TM, single-filament fineness of 3.3 dtex, fiber length of 51 mm, shrinkage rate of 0%, made by Mitsubishi Rayon); then, a pile fabric was formed by sliver knitting, and pile procedures such as brushing, polishing and shearing were conducted on the pile fabric. Accordingly, a step pile fabric was obtained by using the acrylic fiber in Table 1 in the short-pile portion and acrylic fiber (product number: H155 BRE3.3 T51) in the long-pile portion. Evaluation results of the step pile fabric are shown in Table 3.

Comparative Examples 4, 5

Acrylic fibers as shown in Tables 1 and 2 were each prepared the same as in Comparative Example 3 except that the cross-sectional shape of the discharge port of a spinning nozzle was changed.

A step pile fabric was formed the same as in Comparative Example 3 except that the acrylic fiber obtained in Comparative Example 3 was replaced with those obtained in Comparative Examples 4, 5. Evaluation results of the step pile fabrics are shown in Table 3.

In Comparative Examples, the following fibers were used respectively.

-   Comparative Example 1: H156 BHH3.3 TVCL, made by Mitsubishi Rayon. -   Comparative Example 2: V57 BSH3.3 TVCL, made by Mitsubishi Rayon. -   Comparative Example 3: H155 BRE3.3 T38, made by Mitsubishi Rayon. -   Comparative Example 4: V17 BRE3.3 T38, made by Mitsubishi Rayon. -   Comparative Example 5: H180 BRE3.3 T38, made by Mitsubishi Rayon.

TABLE 1 Shrinkage Maximum Rate Value of 2nd before Moment Treatment Single Minimum Value of Area in Boiling Filament Bulkiness Minimum Value Maximum Value of 2nd Moment per Unit Cross-Sectional Water Fineness (mm³/g) × Elongation of 2nd Moment of 2nd Moment of Area per Unit Fineness Shape (%) (dtex) 10¹ (%) of Area (μm⁴) of Area (μm⁴) Fineness (μm⁴) (μm⁴) Example 1 UFO type 29 3.3 0.212 61.1 4223 25035 1280 7586 (requirement I) Example 2 UFO type 30 6.6 0.223 59.2 4103 236005 622 35758 (requirement I) Example 3 triangle type 28 3.3 0.201 50.5 2877 8630 872 2615 (requirement II) Example 4 dumbbell type 30 3.3 0.211 52.2 50868 63585 15415 19268 (requirement III) Example 5 dumbbell type 32 5.6 0.222 55.5 116227 92982 20755 16604 (requirement III) Comp. flat type 25 3.3 0.111 44.2 610 39043 185 11831 Example 1 Comp. round type 35 3.3 0.103 71.2 5150 5150 1561 1561 Example 2 Comp. flat type 0 3.3 0.122 22.5 610 39043 185 11831 Example 3 Comp. round type 0 3.3 0.177 46.2 5150 5150 1561 1561 Example 4 Comp. UFO type 0 3.3 0.181 25.4 4223 25035 1280 7586 Example 5 (requirement I)

TABLE 2 Cross-Sectional Shape of Fiber a b W H Narrowing Flatness Cross-sectional (μm) (μm) a/b (μm) (μm) B/C Degree Degree Shape Example 1 60 6 10 0.024 (=2a/5)  6 (=b) — — — UFO type (requirement I) Example 2 60   7.5  8  0.03 (=1a/2) 10 (=1.7b) — — — UFO type (requirement I) Example 3 — — — — — 0.8 — — triangle type (requirement II) Example 4 — — — — — — 2   — dumbbell type (requirement III) Example 5 — — — — — — 1.6 — dumbbell type (requirement III) Comp. — — — — — — — 7 flat type Example 1 Comp. — — — — — — — — round type Example 2 Comp. — — — — — — — 7 flat type Example 3 Comp. — — — — — — — — round type Example 4 Comp. 60 6 10 0.024 (=2a/5)  6 (=b) — — — UFO type Example 5 (requirement I)

TABLE 3 Type of Raw Pile Length Pile Length Cross-Sectional Stock for Texture Evaluation Rate of Elongation of Short Pile of Long Pile Shape of Short Short Pile (Sensory Test) Recovery from of Short Pile Portion Portion Pile Portion Portion Looseness Softness Bulkiness Compression Portion (%) (mm) (mm) Example 1 UFO type high 5 5 5 44 61.1 5 15 (requirement I) shrinkage Example 2 UFO type high 5 5 5 49 59.2 5 20 (requirement I) shrinkage Example 3 triangle type high 4 5 4 42 50.5 5 15 (requirement II) shrinkage Example 4 dumbbell type high 4 5 5 41 52.2 5 15 (requirement III) shrinkage Example 5 dumbbell type high 4 5 5 40 55.5 5 20 (requirement III) shrinkage Comp. flat type high 2 3 3 23 44.2 5 20 Example 1 shrinkage Comp. round type high 2 3 2 28 71.2 5 15 Example 2 shrinkage Comp. flat type no 2 3 2 22 22.5 10 25 Example 3 shrinkage Comp. round type no 2 3 1 19 46.2 10 25 Example 4 shrinkage Comp. UFO type no 2 3 3 30 25.4 10 25 Example 5 (requirement I) shrinkage

DESCRIPTION OF NUMERICAL REFERENCES

-   1 measuring jig -   10 jig main body -   11˜13 first˜third board members -   14 weight -   15 fiber bundle 

1. A high-shrinkage acrylic fiber, wherein a single-filament fineness is set at 1˜7 dtex, a shrinkage rate is set at 20˜40%, and a post-shrink bulkiness is set at 0.19×10¹˜0.30×10¹ mm³/g.
 2. An acrylic fiber, wherein a single-filament fineness is set at 2˜7 dtex, an elongation rate is set at 50˜70%, and a bulkiness is set at 0.19×10¹˜0.30×10¹ mm³/g.
 3. The acrylic fiber according to claim 2, wherein a minimum value of the second moment of area per unit fineness is at least 600 μm⁴ in a direction where the second moment of area is smallest, and a maximum value of the second moment of area per unit fineness is at least 2200 μm⁴ in a direction where the second moment of area is greatest.
 4. The acrylic fiber according to claim 2, wherein the minimum value of the second moment of area per unit fineness is at least 4000 μm⁴ in a direction where the second moment of area is smallest.
 5. The acrylic fiber according to claim 2, wherein any of requirements (I), (II) and (Ill) described below is satisfied: (I) The cross section of a fiber is formed with a circle and rectangle, where the rectangle having a short side shorter than the diameter of the circle penetrates through the circle in such a way that both ends of the rectangle are positioned outside the circle: when the length of the long side of the rectangle is set as “a” and the length of the short side as “b,” “a/b” is 3˜25; and when the diameter of the circle is set as “W” and the shorter height of the maximum heights measured respectively from each of both long sides of the rectangle inside the circle is set as “H,” “W” is a/10˜4a/5, and “H” is 2/b˜2b. (II) The cross section of a fiber has a triangular shape: when the length of the longest side of the triangle is set as “C” and the height from the longest side of the triangle positioned at the base is set as “B,” B/C is 0.5 or greater. (Ill) The cross section of a fiber has a dumbbell shape and the degree of narrowing (X/Y) is 1.1˜2.5. “X” is the maximum length of major diameter between narrowed ends. “Y” is the length at the narrowest portion.
 6. The acrylic fiber according to claim 2, comprising an acrylonitrile-based polymer containing an acrylonitrile unit at 50 mass % or greater.
 7. A spun yarn, comprising the acrylic fiber according to claim 1 at 20˜50 mass %.
 8. A step pile fabric, comprising: a long-pile portion; and a short-pile portion, wherein the short-pile portion contains the acrylic fiber according to claim 2, and its rate of recovery from compression is set at 35˜90.
 9. The step pile fabric according to claim 8, wherein the amount of acrylic fiber contained in the short-pile portion is set at 20˜50 mass % of the entire pile portion.
 10. The step pile fabric according to claim 8, wherein the base of pile is in the initial state of spun yarn.
 11. The step pile fabric according to claim 8, wherein the length of the short-pile portion is set at 5˜20 mm.
 12. The step pile fabric according to claim 8, wherein the length of the long-pile portion is set at 6˜40 mm.
 13. The step pile fabric according to claim 8, wherein the long-pile portion comprises at least one of acrylic fiber, polyester fiber and animal fiber at 50˜100 mass % of the entire long-pile portion.
 14. The step pile fabric according to claim 8, wherein the long-pile portion is set to have a single-filament fineness of 1˜50 dtex.
 15. The step pile fabric according to claim 8, wherein the cross-section of acrylic fiber or polyester fiber in a fiber axial direction has a flat shape, Y-shape, UFO-shape, or dumbbell shape.
 16. The step pile fabric according to claim 8, wherein the difference in length between the long-pile portion and short-pile portion is set to be 1˜20 mm. 